Chem Unit 2 (Alkenes) Flashcards
Alkenes (homologus series so.. [2 things]
- same functional group (CnH2n)
- same structural formula (C=C)
As you increase chain length…
number of van der waals increases so melting and boiling point increases
Why are alkenes more reactive?
pi bond has delocalised electrons that are hel less tightly and attract positively-charged atoms/groups
Double bond is made up of ?
pi bond and stigma bond
Why are alkenes double-bonded?
unsaturated- can accept hydrogens (still need to make 4 single, covalent bonds, sigma binds (bond 3 atoms covalnetly) still need pi (double bond 1 pi, 1 stigma) to make 4 bonds.
Why di akenes take more energy to break?
pi bond and sigma bond brings carbon atoms closer together- held more tightky
Why can’t there be positional isomers of alkenes?
C=C bond is rigid/planar structure
atoms/grouos attached to it are fixed- so there can’t be positional isomers- Only CHAIN GEOMETRIC ISOMERS
Z isomer.. methyl groups are on..
same side of carbon atom
E isomers..methyl groups are on..
opposite side of carbon atom
What is electrophilic addition?
Electrophiles (positevely charged molecules/atoms ) accept a pair of electrons from the area of high electron density in the double bond (pi bond)
breaks the double bond > turns it intoa single bond (alkane)
Electrophilic addition: why is Bromine a dipol-dipole in the presence of..?
ethene- the double bond of the ethene induces a temporary dipole-dipole
When a hydorgen halide reacts with a unsymmetrical alkene it can make..?
diferent products becuase it depends on which end of the double bond the hakide reacts with e.g. 1-bromo-propane or 2-bromo-propane
The major prodcuct (whats mostrly formed) is going to be the secondary carbocation and the minor product the primary carbocation because?
Secondary carbocation is more stable than primary one
Order: primary>secondary>tertiary
because for primary the c+ is bonded to one other carbon atom (could be a methyl group, halogen atom) , fior secondary 2, tertiary -3
These carbon atoms donate electrons so minimise the positie charge on the carbocation, making it more stable
Add bromine water to alkenes>
alkanes>
alkenes = colourless (unsaturated) alkanes = yellow (saturated)
carbon neutral?
no net carbon emissions into the atmosphere
Making an alkene into an alcohol with sulfuric acid?
conc. H2S04 . alkene
warm it with dilut H2S04 and water > alcohol = H20
sulfuric acid is a catalyst- unchanged but sppeds up rate of reaction so is recycled.
Why is ti hydration?
overall addition of water
Hydrogenation of vegable oils condtions?
- Hydrogen
- heat to 60 degrees
Conditions for producing ehtanol by hydration (steam)
phosphoric acid catalyst- sulfuric acid too corrosive too use on a large scale
fairly low temp = 300 degrees
high pressure = 600-7000 kpa
high concentration of steam
5% intial yield but 95% with recyling of unreacted ehtene and steam
yield for ethanol isn’t 100% by hydration as?
methanol and polyethene are made as side reactions
Advantages of ethanol production by steam?
- quick
- continued recycling of ethene and steam
- no pollution
- high yield
Disadvantages if ethanol production by steam?
- crude oil (non-renewable)
- high pressure (costs)
- high temperature (costs)
polymers are
large molecule made up of thousands of monomers
momomer is..
small molecules that jon together to maje a polymer
addition polymerisation
momomers are added together to make polymers
Why is polyethene fairly unreactive?
100% yield > polyethene no double bond so can’t do much
can do combustion
If polyethene makes LDPE?
softens under high temepratures
branched- can’t pack closely together
used for bags, plastic botttles
If polyethene makes HDPE?
titanium chloride/triethyl aluminium catalyst
60 degrees/atmsopheric pressure
straight-chain alkanes > buckets/crates
poly(propebe) if CH3 ad H groups are arranged facing in same direction…
isotactic (used for car bumpers)
poly(propebe) if CH3 ad H groups are arranged randomly…
actactic (versatile plastic properties mid-way between HDPE and LDPE
Bipol
produced by microbes
eaten by other microbes> decompose it releasing C02 (limited-expensive)
Digesting material
polymer material is digested by microbes (e.g bacteria/fungi)
C=0 (carbonyl group) means tis easier
plymers containing chlorine, benzene or methyl groups are resitant to the microbes.
polymer material that is broken down in the presence of light is..
photodegradable
Co polymers
ethene + CO > Copolymer
C=0 (carbonyl group) absorbs energy from light
increased light energy breaks bonds in copolymers e.g. ETFE
Photodegrading plastics are ideal for plastics that can’t be easily?
recovered (e.g. fishing nets)
If plastics are dumped in landfill they lack…?
light, oxygen, microbes so take a long time to bioddegrade
Main problems with recycling plastics?
- High levels of contamination
- Many types so difficult to sort
mechanical recycling means:
plastic has to be sorted and cleaned first
shredded > melted down into pellets > remoulded into new objects
Chemical recylcling is …?
don’t aghve to be sorted and cleaned
heat and a catalyst ( like thermal cracking) to break long chains into shorter chains and small molecules like ethane and ethene (useful)
If plastics are sorted/cleaned enough it is possible to process them back into..?
their monomers
Thermal recycling?
Equal masses of crude oil and plastic have similair amounts of stored enrgy.
When plastic is burned > energy used to generate steam/electricity
problem with plastics containing chlorine, it releases doxins into atmosphere
3 main way to produce ethanol:
- Hydration by steam
- Using yeast to ferment plant sugars
- using bacteria to produce it from biomass waste
Conditions for thermal cracking?
- High temperature
- nickel catalyst
- High pressure
